Stratigraphy and Petrology of the Lower Eocene Willwood Formation, Bighorn Basin, Wyoming

Abstract

The Willwood Formation (late Paleocene–early Eocene) consists of a complex association of marginal alluvial fan conglomerate grading basinward into stream channel and floodbasin deposits in the Bighorn Basin in northwestern Wyoming. Conglomerate beds are exposed primarily along the western flank of the basin. Conglomerate beds bordering the Beartooth Mountain front contain an assemblage of igneous, metamorphic, and sedimentary rock fragments reflecting a proximal source area. Willwood conglomerate beds along the Absaroka Mountains are composed of quartzite pebbles and cobbles derived from more distant westerly sources and recycled Fort Union Formation sediments. Willwood sandstone units are generally of a subarkose variety, with detrital grains consisting of quartz (68 percent), chert (13 percent), feldspar (10 percent), metaquartz (7 percent), and “heavy” minerals (2 percent). Sandstone textures average fine grained and moderately sorted. Willwood siltstone and clay-stone (mudstone) are mainly quartz, with illite and montmorillinite dominant and kaolinite subordinate among the clay minerals. Vertical changes in Willwood lithology include an upward increase in Precambrian metamorphic detritus, improved textural sorting, and the proportional increase in red beds. Willwood sandstone beds contain large- and small-scale cross-stratification, horizontal stratification, laterally extensive sand bodies indicative of flow regimes, and sediment accumulation processes characteristic of shallow gradient meandering streams. Localized channel fills record chute and neck cut-off during abandonment of a stream channel. Thin sheet sandstone (natural levees) and variegated mud-stone beds of the floodbasin record overbank deposition. Markov chain analysis of Willwood strata demonstrates the presence of repetitive depositional sequences that change at higher stratigraphic levels. Illite and montmorillinite clay minerals in the Willwood Formation, in association with subordinate amounts of kaolinite, indicate the absence of extensive lateritic weathering of source material in adjacent upland areas. Modification of bedrock appears analogous to a red-yellow Mediterranean type of soil formation. Modification of Willwood sediment at the site of deposition was greatly influenced by the development of appreciable alluvial relief through meander belt confinement. Red mud-stone shows low organic carbon amounts coupled with high free iron and manganese, an association expected of a relatively high-standing, well-drained (oxidizing) depositional site. Conversely, greenish-gray (drab) mud-stone shows high organic carbon amounts and low free iron and manganese levels, an association typical of low standing, poorly drained (reducing) sites of deposition. Detailed chemical analysis of a selected mudstone unit shows profile development of free iron, aluminum, and manganese, and unusually high levels of organic carbon for red mudstone. Such a unit is interpreted to be an immature soil developed on the Willwood landscape through leaching of mobile constituents and organic matter concentration.